Certain types of glass have optical properties that can be altered when they are exposed to radiation. In particular, some index of refraction variations can be permanently inscribed in these types of glass with ultraviolet radiation. In Bragg grating writing by flood exposure, an interference pattern is permanently written in the form of index of refraction variations in an optical fiber. Typically, a laser beam in the ultraviolet (UV) region of the optical spectrum is split into two sub-beams. The two sub-beams are then recombined to produce an interference pattern which is shone on the core of the optical fiber for a period of time. After the laser beam is turned off, the index of refraction variations stay inscribed in the optical fiber.
PCT Application 00/02068 filed on Jun. 30, 1999 by Bhatia et al. describes an apparatus to write Bragg gratings in an optical fiber. The apparatus includes a laser, which produces a laser beam. The laser beam is split into two sub-beams by a beam splitter. Then, the two sub-beams are each reflected by a plurality of mirrors to make them converge at a certain location in space. The two converging beams interfere and therefore produce an interference pattern at the certain location. The optical fiber is positioned at the certain location to write the grating.
Apparatuses such as the one described in the above-referenced PCT application present many disadvantages. First, the mirrors have to be precisely aligned to produce the desired interference pattern. Also, the whole apparatus has to be very rigid and isolated from external vibrations to keep the interference pattern at a precise location in space. If the interference pattern is displaced during the writing process, the grating will be veiled and may eventually be useless. In addition, the surface of the mirrors has to be kept clean in order to bring as much energy as possible to the certain location where the interference pattern is produced.
Two properties that are often required of Bragg gratings are apodization and balance. Apodization relates to having an interference pattern including a plurality of bright fringes and a plurality of dark fringes wherein the bright fringes are not uniformly bright across the whole interference pattern. Therefore, if an apodized interference pattern with fringes having a low intensity close to the extremities of the grating is used to produce the Bragg grating, the index of refraction differences will also be apodized, which is desirable in some Bragg gratings used as optical filters. Balance relates to having indices of refraction in the grating which vary alternatively above and below an average value. In the apparatus described above, only variations in indices of refraction in one direction are possible as dark fringes in the interference pattern produce no variation in the index of refraction inside the optical fiber and bright fringes all produce variations in the index of refraction inside the optical fiber having a same sign. Once again, it is often desirable to have variations above and below an average value when the gratings are used as optical filters.
To produce a balanced grating, two exposures are required in the apparatus described above. In a first exposure, the beams, eventually apodized, are used to create the variations in index of refraction as described above. In the second exposure, the sub-beams are slightly offset to provide a uniform increase in index of refraction along the grating. However, this two-step process is time consuming as two exposures have to be made. In addition, the apodization is performed through collimators and spatial filters which need to be precisely aligned with the rest of the apparatus.
In view of the above, there is a need in the industry to provide new apparatuses and method for writing features in or on a photosensitive medium.
More particularly, the invention relates to the use of an interference pattern between two coherent light beams to induce changes in the index of refraction of the medium wherein the two light beams are produced by splitting a first light beam and propagated in a prism through total internal reflection.